The Hyborian Age does not take place in prehistory, but in an alternate history where cultures are compressed down to a degree that loses its true historicism. Picts, for instance, are actually native Americans, while for some reason being named after the Celtic peoples of north-western Europe (if we are being historians here, R.E.H. should never have written Picts into his Eurasian adventures in the first place). Darfari, for some reason, are cannibals who file their teeth, which is likely a relic of the racism of the time, and a shame that African people weren’t more accurately represented, especially as they had some of the largest kingdoms in the prehistorical time. R.E.H. was a pulp fiction writer, but a historian he certainly was not.
If there were a readers’ interest in ancient Japanese culture (which is often simplified today as ‘samurais and ninjas and fortresses’) then I have no doubt he would have written a story concerning samurais and ninjas and fortresses. That would, of course, include the unique Japanese fortress style that most consider synonymous with anything regarding ancient or feudal Japan. Despite the fact that the timeline wouldn’t reflect the real-world history. History is not a barrier to R.E.H.
Part of being a pulp writer meant that an author would be using the themes given to them by their publishers. When his publishers told him to write a story about the Wild West, he did. Or boxing tales. Or tales about fictional puritans, or Atlantean kings. If they had suggested a Japanese story, he would have likely told one of Yamatai, of samurai, fortresses, and feudal wars, because that is the most vivid understanding of Japanese history in American popular culture.
Another significant piece that you’re missing here is that the players requested exactly this type of architecture. Actually, as it turns out, I suggested it back in March '17, and people were receptive: Suggestion for new or "missing" building tiles
You can try and tell them that they got it “wrong”, but honestly, so long as people enjoy it, they’re doing it right. Both with the (beautiful, IMO) Pict set, and with this new Yamatai set. R.E.H. was no historian, and Conan Exiles shouldn’t be held up to those standards when the original works did not reflect history with anything more than pulp accuracy.